The stability of xenotime in high Ca and Ca-Na systems, under experimental conditions of 250-350°C and 200-400 MPa : the implications for fluid-mediated low-temperature processes in granitic rocks

The stability of xenotime was tested by experiments in the presence of a silicate mineral assemblage and two different fluids, 2M Ca(OH)_2 or Na_2Si_2O_5 + H_2O, under P-T conditions of 200-400 MPa and 250-350°C. The xenotime was stable in runs with 2M Ca(OH)_2, replicating the low-temperature metasomatic alterations of granitic rocks, except in experiment at 350°C and 400 MPa, where some (Y,REE)-rich fluorapatite formed. Experiments with Na_2Si_2O_5 + H_2O resulted in significant xenotime alteration and partial replacement by an unknown (Y,HREE)-rich silicate, and in the formation of minor amounts of (Y,REE)-rich fluorapatite. The latter indicate preferential partitioning of Y and REE into silicates over phosphates during low-temperature, metasomatic processes in a high Na-Ca system, similar to peralkaline granitic rocks

Stability of monazite and disturbance of the Th-U-Pb system under experimental conditions of 250-350^{\circ}C and 200-400 MPa

This experimental study provides important data filling the gap in our knowledge on monazite stability under conditions of fluid-mediated low-temperature metamorphic alteration and post-magmatic hydrothermal alterations. The stability of monazite and maintenance of original Th-U-total Pb ages were tested experimentally under P-T conditions of 250-350 ^{\circ}C and 200-400 MPa over 20-40 days. The starting materials included the Burnet monazite + K-feldspar ± albite ± labradorite + muscovite + biotite + SiO_2 + CaF_2 and 2M Ca(OH)_2 or Na_2Si_2O_5 + H_2O fluid. In the runs with 2M Ca(OH)_2, monazite was unaltered. REE-enriched apatite formed at 350 ^{\circ}C and 400 MPa. The presence of the Na_2Si_2O_5 + H_2O fluid promoted the strong alteration of monazite, the formation of secondary REE-enriched apatite to fluorcalciobritholite, and the formation of REE-rich steacyite. Monazite alteration included the newly developed porosity, patchy zoning, and partial replacement by REE-rich steacyite. The unaltered domains of monazite maintained the composition of the Burnet monazite and its age of (or close to) ca. 1072 Ma, while the altered domains showed random dates in the intervals of 375-771 Ma (250 ^{\circ}C, 200 MPa run), 82-253 Ma (350 ^{\circ}C, 200 MPa), and 95-635 Ma (350 ^{\circ}C, 400 MPa). The compositional alteration and disturbance of the Th-U-Pb system resulted from fluid-mediated coupled dissolution-reprecipitation. In nature, such age disturbance in monazite can be attributed to post-magmatic alteration in granitic rocks or to metasomatic alteration during metamorphism. Recognition of potentially altered domains (dark patches in high-contrast BSE-imaging, developed porosity or inclusions of secondary minerals) is crucial to the application of Th-U-Pb geochronology

Apatite from NWA 10153 and NWA 10645 : the key to deciphering magmatic and fluid evolution history in Nakhlites

Apatites from Martian nakhlites NWA 10153 and NWA 10645 were used to obtain insight into their crystallization environment and the subsequent postcrystallization evolution path. The research results acquired using multi-tool analyses show distinctive transformation processes that were not fully completed. The crystallization history of three apatite generations (OH-bearing, Cl-rich fluorapatite as well as OH-poor, F-rich chlorapatite and fluorapatite) were reconstructed using transmission electron microscopy and geochemical analyses. Magmatic OH-bearing, Cl-rich fluorapatite changed its primary composition and evolved toward OH-poor, F-rich chlorapatite because of its interaction with fluids. Degassing of restitic magma causes fluorapatite crystallization, which shows a strong structural affinity for the last episode of system evolution. In addition to the three apatite generations, a fourth amorphous phase of calcium phosphate has been identified with Raman spectroscopy. This amorphous phase may be considered a transition phase between magmatic and hydrothermal phases. It may give insight into the dissolution process of magmatic phosphates, help in processing reconstruction, and allow to decipher mineral interactions with hydrothermal fluids

Mineralogy of Cobalt-Rich Ferromanganese Crusts from the Perth Abyssal Plain (E Indian Ocean)

Mineralogy of phosphatized and zeolitized hydrogenous cobalt-rich ferromanganese crusts from Dirck Hartog Ridge (DHR), the Perth Abyssal Plain (PAP), formed on an altered basaltic substrate, is described. Detail studies of crusts were conducted using optical transmitted light microscopy, X-ray Powder Diffraction (XRD) and Energy Dispersive X-ray Fluorescence (EDXRF), Differential Thermal Analysis (DTA) and Electron Probe Microanalysis (EPMA). The major Fe-Mn mineral phases that form DHR crusts are low-crystalline vernadite, asbolane and a feroxyhyte-ferrihydrite mixture. Accessory minerals are Ca-hydroxyapatite, zeolites (Na-phillipsite, chabazite, heulandite-clinoptilolite), glauconite and several clay minerals (Fe-smectite, nontronite, celadonite) are identified in the basalt-crust border zone. The highest Ni, Cu and Co contents are observed in asbolane and Mn-(Fe) vernadite. There is significant enrichment of Ti in feroxyhyte−ferrihydrite and vernadite. The highest rare earth element (REE) content is measured in the phosphate minerals, less in phyllosilicates and Na-phillipsite. The geochemical composition of minerals in the DHR crusts supports the formation of crusts by initial alteration, phosphatization and zeolitization of the substrate basalts followed by oscillatory Fe-Mn oxyhydroxides precipitation of hydrogenous vernadite (oxic conditions) and diagenous asbolane (suboxic conditions)

Metal leakage from orthodontic appliances chemically alters enamel surface during experimental in vitro simulated treatment

Abstract Human enamel is composed mainly of apatite. This mineral of sorption properties is susceptible to chemical changes, which in turn affect its resistance to dissolution. This study aimed to investigate whether metal leakage from orthodontic appliances chemically alters the enamel surface during an in vitro simulated orthodontic treatment. Totally 107 human enamel samples were subjected to the simulation involving metal appliances and cyclic pH fluctuations over a period of 12 months in four complimentary experiments. The average concentrations and distribution of Fe, Cr, Ni, Ti and Cu within the enamel before and after the experiments were examined using ICP‒MS and LA‒ICP‒MS techniques. The samples exposed to the interaction with metal appliances exhibited a significant increase in average Fe, Cr and Ni (Kruskal–Wallis, p < 0.002) content in comparison to the control group. The outer layer, narrow fissures and points of contact with the metal components showed increased concentrations of Fe, Ti, Ni and Cr after simulated treatment, conversely to the enamel sealed with an adhesive system. It has been concluded that metal leakage from orthodontic appliances chemically alters enamel surface and microlesions during experimental in vitro simulated treatment

High-temperature fluids in granites during the Neoarchaean-Palaeoproterozoic transition: Insight from Closepet titanite chemistry and U-Pb dating (Dharwar craton, India)

International audienceThe aim of this paper is to determine the composition and age of high-temperature (HT) hydrothermal fluids in Closepet granites (Dharwar craton, India). The magmatic origin of the Neoarchaean Closepet batholith has been recognized and dated many times, whereas a HT fluid system, strong evidence for which has been partly investigated, has never been dated. Titanite was chosen for a geochemical study on its pristine and altered domains to illustrate the fluid composition; the domains were dated to recognize the timing of fluid activity. Titanite presents distinct trace element enrichments in non-altered domains and strong depletions in trace elements in marginal, irregular, patchy zones. The high total rare earth element (REE) concentrations and the marked negative Eu anomalies suggest crystallization from a residual melt. The negative Eu anomalies diminish progressively in the REE patterns of altered parts and become positive. Trace element ratios (Nb/Ta, Y/Ho, and U/Th) indicate fluid-crystal interactions. Strong U/Th fractionation might be linked to the presence of CO2 and Cl-. Zr-in-titanite thermometry yields a crystallization/recrystallization temperature of ~700 °C for both magmatic and altered domains. The titanite factor Ti+4/(Al+3 + Fe+3) reveals an affinity with significant mantle input to the fluid system. Pristine and altered titanite domains have been successfully dated by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Reversely discordant apparent ages suggest interaction with fluids, which modified the Pb composition of the magmatic titanite. The oldest semi-concordant analyses of titanite cores indicate an age of approximately 2500 Ma, which we interpret as the minimum age of magmatic titanite crystallization. Narrow dark rims of titanite with considerably lower Th/U ratios form a more coherent group of analyses defining an upper concordia intercept age of 2345 ± 0.016 Ma, interpreted as the best estimate for the time of complete titanite resetting associated with the metasomatic event. The HT fluid system appears to have remained active into the Palaeoproterozoic. Confirmation by dating that the tested fluids belonged to the Archaean hydrothermal system gives us information about this system at the time, but the information that it worked at least until the Palaeoproterozoic provides additional valuable information about the hydrothermal systems during the Neoarchaean-Palaeoproterozoic transition